What does it take for a material to be transparent to light?

I need help exploring this question: what makes a solid transparent/opaque?
If I have clear sheet of plastic and I introduce imperfections (such as small air bubbles inside), it turns white if the # of imperfections is large. Is it possible to maintain the polymer transparent with such air pockets inside? If bubbles are made very small (say taking the limit as diameter -> 0), would that do the trick?

I'm interested in visible light, so frequency is constrained. What would it take to make a thermoplastic polymer with impurities be transparent to visible light frequencies? If I can make the air bubbles very small (few nm), would that do the trick regardless of impurity shape and density (number of bubbles per unit volume).
Are there any theories/papers on this subject that may be helpful?

I'm interested in visible light, so frequency is constrained. What would it take to make a thermoplastic polymer with impurities be transparent to visible light frequencies? If I can make the air bubbles very small (few nm), would that do the trick regardless of impurity shape and density (number of bubbles per unit volume).
Are there any theories/papers on this subject that may be helpful?

What? If this were true, what would be the point of sunscreen? If anything, it would be lower frequencies that are more transparent, at least from what I've seen. Would I need to worry about gamma rays from a nearby supernova?

What? If this were true, what would be the point of sunscreen? If anything, it would be lower frequencies that are more transparent, at least from what I've seen.

Depends on what material. Lower frequencies are more transparent to dielectrics but have no hope of going through conductors. When I say low frequencies I mean frequencies below the plasmon frequency. For frequencies above the plasmon frequency the metal is transparent.

Sunscreen is special in that it has molecular transitions in the near UV energy range that gets through to earth.

A porous but not absorbing material appears white as the light scatters on the pores. That can be reflection at the interface of the bubble and the matrix, or diffraction on the small imperfection. The rays coming from a light source will not travel in the original direction, you can not see it through the sheet of material. The scattering depends on the relative size of the pores with respect to the wavelength. Bubbles, size of a few nanometer do not scatter visible light appreciably. Scattering also depends on the refractive indices of the material filling the bubbles with respect to the matrix itself. White paper is not transparent, but transparency improves if you soak it in oil.